The Microsoft(MSFT) co-founder is currently the world's richest man, according to several different rankings.
After leaving full-time employment at the company in 2008 (he's still an adviser), most of his time is now spent on the Bill and Melinda Gates Foundation, his charity which focuses on global development and health issues.
Source: *Most nominal wealth estimates are from The Wealthy 100 by Michael Klepper and Robert Gunther, and include wealth at time of death, including bequests. Estimates were then calculated as a fraction of the overall economy at time of death by Sam Williamson of Measuring Worth, and then presented in 2013 dollars based on the size of the current U.S. economy. Estimates for Gates and Buffett provided by Wealth-X.
Biographical information from The Wealthy 100 by Michael Klepper and Robert Gunther, Encyclopedia Britannica, and The Prize by Daniel Yergin.